1. Field of the Invention
The present invention generally relates to ion implantation processes for manufacture of semiconductor integrated circuit devices and, more particularly, to ion implantation for creating buried layers such as silicon-on-insulator (SOI) substrates.
2. Description of the Prior Art
The art of semiconductor electronic device manufacture has become highly sophisticated in recent years to provide a wide range of electrical properties of the devices, often at very high integration density. The capability to determine the electrical properties with high reliability, consistency and manufacturing yield is often limited, as a practical matter, by the tools used for processing the semiconductor material, usually in the form of a wafer. Such tools are often complex and of high precision. Therefore such tools are generally expensive to build and maintain. The principal expense of modern semiconductor devices is thus a portion of the cost of the tools used to produce them and, therefore, varies inversely with tool throughput. Of course, tool cost of each manufactured unit also is proportional to the cost and complexity and maintenance costs of each of the tools used in its fabrication.
An example of such a process is formation of semiconductor-on-insulator (SOI) substrates. SOI structures can be created by depositing or growing an insulator, (e.g. oxide) on a substrate followed by epitaxial growth or deposition of a further layer of semiconductor. Annealing may be required to develop a monocrystalline structure in the further semiconductor layer. This process is complex, proceeds slowly (and is thus expensive) and is of relatively low yield (further increasing unit cost) since the monocrystalline layer must be substantially free of crystal lattice dislocations; a quality which is difficult to achieve over oxide.
A preferred technique is to form the insulator by implantation of ions into a monocrystalline wafer such that the oxygen ions combine with the substrate material at a desired depth within the wafer to form the buried insulator while leaving the surface monocrystalline layer substantially intact. However, to obtain an ion beam of sufficient purity (e.g. free of ions of other than an intended element or radical) mass analysis is often employed and results in loss of a large fraction of the beam current.
Basically, mass analysis involves passing the ion beam through a dipole magnetic field established across a small gap. Assuming substantially the same velocity of the charged particles in the ion beam, the magnetic field will exert a force on each ion in a direction in accordance with the charge thereon perpendicular to both the direction of motion and the magnetic field. This force results in an acceleration inversely proportional to the square root of the ion (or electron) mass and ions having differing charge or differing mass are placed on different trajectories. Thus, undesired ions can be intercepted and removed from the ion beam resulting in a beam populated by only identical ions.
However, this process also causes loss of significant populations of desired ions from the beam through several mechanisms such as spreading of the beam within the dipole magnet in the direction of the pole pieces such that a significant number of ions strike the faces thereof. Further, the mass analysis process results in a beam of relatively small cross-section which must be scanned across the wafer or workpiece and electrical or magnetic deflection of the ion beam results in further loss of ions from the mass-analyzed beam by similar mechanisms. Perhaps most significantly for oxygen (which is the material of choice for forming SOI structures), ions may be produced in several forms which will be affected differently by the mass analysis process: Oxe2x88x92 and O+ ions will be separated by charge (and, generally, velocity) and Oxe2x88x92 and O2xe2x88x92 ions will be separated by mass.
Accordingly, due to the relatively low beam current and the relatively high concentration of ions which must be delivered to form an insulator of significant thickness and adequate integrity, the ion implantation process proceeds slowly and throughput is very low, resulting in very high tool costs which dominate the unit cost of SOI structures. It is estimated that an 80% reduction in such costs could be achieved through increase of ion beam current. Moreover, while SOI structures are known to have some important performance advantages over other known semiconductor technologies, those advantages cannot be economically exploited without achievement of a substantial portion of that cost reduction.
In an effort to obtain increased ion beam currents (and wider beam cross-section to require, at most, mechanical scanning to avoid loss of ions to electrical or magnetic deflection arrangements), a technique referred to as plasma source ion implantation (PSII) has been developed. This technique involves production of a plasma near or in contact with a surface of a workpiece and pulsing a voltage between the workpiece and the plasma to accelerate the ions to achieve the desired implantation energy and implantation depth.
Pulsing of the accelerating voltage is necessary in order to avoid arc breakdown between the workpiece and the plasma. Pulsing, of course, also reduces the duty cycle of the implantation process and thus limits the speed at which the implantation process could proceed. In general, the time period between pulses cannot be reduced beyond a particular limit determined by other parameters of the process such as ion and electron mobility in order to avoid arc breakdown.
Unfortunately, this technique has been found unsuitable for forming SOI structures since it causes excessive damage to the crystal lattice near the surface of the workpiece beyond practical recovery through subsequent annealing. It has been theorized (see xe2x80x9cBoron Doping of Silicon by Plasma Source Implantationxe2x80x9d by R. J. Matyi et al., Surface and Coating Technology 93 (1997), pp. 247-253) that the damage is due to ions implanted at the beginning and end of the accelerating voltage pulse having lower energy and which are thus implanted at reduced depth. This observation also implies that PSII would also be unsuitable for producing any buried layer in which the concentration of buried layer material in the overlying layer is at all critical to intended device function (e.g. a buried conductive plate in a memory device). Some damage may be done in the surface layer by contact with the plasma, as well.
Electron-cyclotron resonance (ECR) plasma sources are known in the art and operate well at relatively high vacuum levels. In these sources microwave energy is projected through a dielectric window to form an electromagnetic wave in a plasma chamber in which a strong magnetic field is present. A gas to form the plasma is introduced at an inlet and, as the gas is ionized, electrons are directed in circular trajectories around the magnetic field lines. When the electrons are rotating with the same frequency as the microwave power, the microwave power increases their energy. The electrons will absorb more energy at high vacuum (fractions of a mTorr) since fewer collisions will occur. Therefore, ECR plasma sources are used to facilitate generation of a plasma.
Also, due to the high energy of electrons in the ECR system, molecules of gases from which the plasma is formed are readily cracked to their atomic species. It should be recognized that the number of ions striking the chamber, being neutralized, returning to the gas and again being ionized is many times greater than the number of molecules (or atoms) of gas provided to the plasma/reaction chamber. Therefore, there will be many collisions between high energy electrons and gas molecules or molecular ions (e.g. O2+) to cause gas cracking. It is also known that, by virtue of this mechanism of cracking of molecules of gases, when oxygen is used to form the plasma that a preponderance of O+ ions will be produced.
Plasma sources with surface magnetic fields are also known and generally referred to as xe2x80x9cbucketxe2x80x9d sources. These plasma sources may also be driven with inductively coupled power In these sources, the electric field is produced by an antenna at the dielectric window. A magnetic field is provided at the surface of the plasma generation chamber by alternating polarity pole pieces surrounding the chamber. The plasma generation action is similar to the ECR plasma source described above except that the electrons are not resonant around magnetic field lines.
It should also be recognized that use of a high vacuum with either of the sources allows undesired materials to be largely removed from the plasma. That is, the reaction chamber is substantially evacuated and the desired material for the plasma introduced into the chamber, usually in a highly pure form. Further, a number of known filtration arrangements are known and can be used in the path of the gas from the source to the reaction chamber inlet to remove particular contaminants such as moisture. Therefore, a plasma can be provided which is substantially free of ions of contaminant materials.
Additionally, it is known to provide a magnetic filter in a plasma chamber to further increase Oxe2x88x92 and O2xe2x88x92 production. When oxygen is used to form the plasma, Oxe2x88x92 and O2xe2x88x92 ions can be preferentially produced by a bucket source with a magnetic filter.
Magnetic filters are generally formed by a structure which provides a magnetic field across the path along which ions are extracted from the plasma. A permanent magnet or electromagnet in the form of a rod within the chamber, permanent magnets or electromagnets placed with poles of opposite polarity across the chamber or a high current between apertures of a multi-aperture extraction grid are examples of known structures that can function as magnetic filters. In this regard, it should be appreciated that early ECR plasma sources used for ion implantation used an apertured extractor and produced a narrow beam since broad sources are difficult to mass analyze and tend to develop overlapping patterns of ion species across broad targets.
While ECR and bucket sources can produce high density plasmas of relatively high purity even without use of a magnetic filter, their principal application in semiconductor processes is for surface treatments such as deposition or etching rather than implantation. In fact, such sources can be used for a PSII process as discussed above to improve purity of the population of ions produced and implanted. However, no solution has been found for the unacceptable amount of crystal lattice damage which results from broadened ion energy distribution during pulsed implantation to form a buried layer which is characteristic of PSII processes. Otherwise, only marginal increases in ion beam current and beam width can be achieved within the present state of the art and which do not support significant increases in tool throughput and cost reduction.
It is therefore an object of the present invention to provide a plasma tool which is capable of producing a large current of ions of high purity and of limited distribution of energies.
It is another object of the invention to provide a technique of ion implantation which proceeds with greatly enhanced speed and a tool with which the technique can be practiced.
It is a further object of the invention to provide a tools and method for producing SOI substrates and other structures having a sharply defined buried layer with high throughput and at low tool cost.
It is yet another object of the invention to provide a simplified and space-efficient ion implantation tool.
It is another further object of the invention to provide a tool and technique of ion implantation which proceeds with substantially increased speed and can be performed continuously.
In order to accomplish these and other objects of the invention, a method of forming a buried layer in a body of semiconductor material is provided including steps of providing an acceleration voltage corresponding to a desired depth of the buried layer within the body of semiconductor material adjacent a source of ions, and ionizing a material subsequent while the acceleration voltage is present to provide a narrow and well-controlled distribution of ion implantation energies.
In accordance with another aspect of the invention, an ion implantation apparatus is provided including an ion source, an arrangement for applying an accelerating voltage field adjacent the ion source, and an arrangement for controlling said ion source,to provide ions only when said acceleration voltage field is present adjacent said ion source.
In accordance with a further aspect of the invention, a semiconductor structure such as a semiconductor-on-insulator wafer is provided having a well-defined buried layer formed from a first species of ions of an implanted material wherein a second species of ions of the implanted material are implanted at a depth beyond the buried layer.